USB:Architectural Overview

Ref: Chapter - 4 of USB 2.0 Specs by USB-IF

The USB is a cable bus that supports data exchange between a host computer and a wide range of simultaneously accessible peripherals. The attached peripherals share USB bandwidth through a host scheduled, token-based protocol. The bus allows peripherals to be attached, configured, used, and detached while the host and other peripherals are in operation.



USB System Description


A USB system is described by three definitional areas:

  1. USB interconnect
  2. USB devices
  3. USB host


USB interconnect


The USB interconnect is the manner in which USB devices are connected to and communicate with the host. This includes the following:


  • Bus Topology: Connection model between USB devices and the host.
  • Inter-layer Relationships: In terms of a capability stack, the USB tasks that are performed at each layer in the system.
  • Data Flow Models: The manner in which data moves in the system over the USB between producers and consumers.
  • USB Schedule: The USB provides a shared interconnect. Access to the interconnection is scheduled in order to support isochronous data transfers and to eliminate arbitration overhead.


Bus Topology


The USB connects USB devices with the USB host. The USB physical interconnect is a tiered star topology. A hub is at the center of each star. Each wire segment is a point-to-point connection between the host and a hub or function, or a hub connected to another hub or function. Due to timing constraints allowed for hub and cable propagation times, the maximum number of tiers allowed is seven (including the root tier).


Fig: USB Bus Topology


USB Host


There is only one host in any USB system. The USB interface to the host computer system is referred to as the Host Controller. The Host Controller may be implemented in a combination of hardware, firmware, or software. A root hub is integrated within the host system to provide one or more attachment points.


USB Devices


USB devices are one of the following:

  • Hubs, which provide additional attachment points to the USB
  • Functions, which provide capabilities to the system, such as an ISDN connection, a digital joystick, or speakers


USB devices present a standard USB interface in terms of the following:

  • Their comprehension of the USB protocol
  • Their response to standard USB operations, such as configuration and reset
  • Their standard capability descriptive information